Abstract：Identification of friction data at high temperatures is still a critical point in the optimization of forming processes. In the present paper the WHUST (Warm and Hot Upsetting Sliding Test) dedicated to the tribology of metal forming processes at high temperature is presented. The friction test is able to operate according to a large range of plastic strains, contact pressures and temperatures. The methodology to use the friction test to respect contact conditions encountered at the tool/workpiece interface of various industrial forming processes is discussed. Then three designs of experiments are achieved. The first set of experiments is performed to study the effects of the composition of graphite based lubricants on friction of hot forging tool. A particular attention is paid on the volume of lubricant used, the size of graphite particles and the amount of binding agents. The second set of experiments is performed to quantify the ability of two white lubricants to reduce friction and protect tool surface. The first lubricant is a mineral salt; the second one is an organic salt. Tests are performed at 1200 °C on AISI 4820 steel specimens, with AISI H11 nitrided steel contactors. Results show the tested white lubricants lead to coefficient of friction in the same range of the graphite lubricant one, but white lubricants lose their ability to reduce friction as soon as the sliding lengths becomes greater to 10 mm, where graphite lubricants can undergo sliding length greater than 30 mm. The last experiments are performed to qualify a new sol-gel coating deposited on a common tool steel AISI H13. Tests are operated at 1100 °C on AISI 4820 steel specimens. After each test, the ability of the coating to reduce friction and prevent metal transfer from specimens to the coated tool surfaces is established.

Keywords: Hot formging; Tribology; White lubricant; Friction coefficient; coatings.

1. Introduction

Lubricants used in metal forming ensure multiple functions. Obviously they have to reduce friction, but they also have to cool down tool surface, to protect workpiece surface from scratch or cracks, to limit surface pollution by wear debris and, most of all, they have to respect the environment [1]. This large range of functionalities has encouraged suppliers to develop numerous families of lubricants: liquid lubricants, containing water or oil, solid lubricants, such as soaps, phosphates, polymers or mineral salts. Even if the chemical compositions of all those lubricants may be totally different, most of them are the association of three main chemical parts: an active agent to reduce friction and protect tool and workpiece surfaces, a binder to help the lubricant to adhere to surfaces and to be able to undergo large deformation, and a conveyor to “drive” the active agents to the contact surfaces   [2].

On the other hand, tribology of metal forming process is quite complex. Friction between tools and workpieces depends on numerous factors:

x Mechanical parameters such as contact pressure, plastic strain, strain rate, sliding velocities, material yield stress,x     Physical parameters such as roughness, temperature, viscosity, lubricant film  thickness,x Chemical parameters such as surface energy, presence of oxides, chemical reactions between lubricant and metallic surfaces, etc.

The choice of a lubricant is still a crucial point in the development of a metal forming process. A  non- appropriated lubricant, or a non-appropriated use of a good lubricant, may decrease tool life by generating an unwanted wear regime, or may lead to a poor workpiece surface quality. The difficulties of choosing a lubricant arise, first, from the great number of lubricants available on the market and, second, from the difficulty to test them according to the real conditions of use they are submitted  to.

The present work proposes to improve the understanding of lubricated contact during hot forging of metals by the use of a specific friction test. First, the friction test and the methodology developed to analyse its results will be presented. Then, the methodology will be applied to several industrial cases, involving graphite based lubricants, white lubricants and new sol-gel tool  coatings.